Moulded resistors and electrical switches



Dec. 1, 1959 v. A. w. PAGE ETAL 2,915,728

MOULDED RESISTORS AND ELECTRICAL SWITCHES Filed Dec. 2, 1957 2 Sheets-Sheet l I F g.2. 2

Dec. 1, 1959 PAGE 2,915,728

MOULDED RESISTORS AND ELECTRICAL SWITCHES Filid D00. 2, 1957 2 Sheets-Sheet 2 United States Patent 2,915,728 MOULDED RFSISTORS AND ELECTRICAL SWITCHES Victor Arthur William Page, Hove, and John Robert Clark, Swindon, England, assignors to The Plessey Company Limited, Ilford, Essex, England, a British company Application December 2, 1957, Serial No. 700,233 Claims priority, application Great Britain December 6, 1956 2 Claims. (Cl. 338-190) This invention relates to electrical connecting devices and to moulded resistors equipped therewith. It is particularly, though not exclusively, applicable to the connections required on resistors having resistor elements moulded from thermo-setting resin, especially phenolformaldehyde resin, which has been rendered conductive to a desired degree by mixing carbon powder with the resin moulding material.

Another application consists in electrical commutator switches.

According to the invention a connecting device comprises a contact or terminal element formed of a moulded mixture of a metal powder with a resin acting as a binder, the mixture being such that the moulded contact or connector elements are highly conductive.

From another aspect, an electrical contact device comprises a moulded plastic base having embedded in one surface thereof contact elements moulded of a mixture of a metal powder and a resinous binder, said contact elements being moulded flush with the said surface of the base.

Some difficulty has hitherto been experienced with moulded resistors in ensuring reliable low-resistance electrical connections, both for permanent inclusion of the resistor in an electric circuit and Where a contact brush is required to provide a sliding contact as avariable tapping movable along the resistor. With a view to reducing this difficulty, another aspect of the invention consists in a moulded resistor element which is provided, at the points at which electrical contact connection is required, with integrally moulded resin-bound low resistance contact portions, said portions consisting of a mixture of a resin identical with or similar to that con-,

tained in the resistor element, and a metal powder. The similarity of the resins should be such that the two resins can be made to adhere firmly to each other without the use of additional adhesive matter. In order to obtain integral moulding where a thermo-setting resin is employed, the contact portions must, when a thermo-setting resin is employed, be joined to the resistive element before or during the setting of the resin so as to become integral parts of the resistor elements. I

In the case of resistive elements comprising a mixture of carbon and phenol-formaldehyde resin such as the material known under the R.T.M. Bakelite, the contact portions preferably consist of a mixture of approximate- 1y 75% by weight of silver powder and 25% by weight of the resin moulding powder. The mixture ratio should preferably be ascertained by experiment for each particular brand of moulding powder and for the grain size of the silver powder employed, since satisfactory results will generally only be obtained within a few percent of the optimum mixture. In the case of a fixed connection, the end of a metal terminal may advantageously be embedded into the contact portion.

In the case of resistor tracks for co-operation with a wiper or brush, the moulded contact portions may constitute the end portions of the track and serve as flush end contacts.

Similarly one or more tapping-contacts containing ice metal powder may be formed in the track ofa moulded track resistor, flush with the surface of the latter, to provide a tapping point or tapping points of low contact resistance. v

Alternatively the contacts may beincorporated in a non-conductive plastic surface which may cover the track, or be arranged laterally of the latter, their matrices being integral with both the matrix of the track and the material of the covering or the like. Such contacts have been found to be superior, as regards wear, to metal contacts embedded in a plastic body.

Another aspect of the inventionconsists therefore in a commutator switch in which the switch contacts consist of a moulded mixture of a thermoset resin forming. a matrix and a suitable metal forming aconduc tive skeleton in this matrix, such' contacts being embedded in an insulating body of the thermos'et resin integral with the said matrix, to provide a continuous smoothcontact surface including conductive contact portions separated and insulated from each other by said insulating body.

A contact brush'for co-operating with contacts according to the invention may preferably consist of amoulded mixturesimilar to that of the contacts, 'but, other brushes or wipers, for example of carbon or of 'suitable metal, may alternatively be employed.

In the drawing, v p I Figure 1 is a sectional elevation of a potentiometer according to the invention having an open resistor track,

Figure 2 similarly shows one end of apotentiom'eter of modified construction,

Figure 3 is a section of a further modified form, in which the resistor track itself is covered with insulating material, and I 1 1 Figure 4 similarly illustrates a'commutator switch,

Figure 5 is an axial section of'astepp'ed attenuator incorporating. connecting elements according to the invention,

Figure 6 is a somewhat'schematicplan view of the base showing the resistor track and'contact elements, while 1 Figures 7 and 8 illustrate a modified which may replace that of Figure 6,

Figure 7 being a sectional elevation, and Figure "8 being a front .viewthereof.

Referring now to the drawing,=Figure '1 illustrates a potentiometerhaving an open resistor track 1. The track form of base,

is moulded of a mixture of carbon and phenol-formaldehyde moulding powder, andis "embedded in a surface groove of an insulating base 2. Thebase is moulded of the same moulding powder which is incorporated in the track, but instead of carbon, contains anon-conductive is applied to cure the resin, whereby the resin of the various'parts is united into one integral matrix body, in

the appropriate parts of which the carbon andlthe silver are respectively embodied. 6 is a contact brush adapted to move along the track, as indicated by the arrow, to form avariable tapping for the potentiometer, 7 being a flexible conductor. Assuming that a predetermined potential difference is applied between the terminals '4, movement of the brush 6 will provide 'a continuouslyvariable potential at the tapping. While, however, the contact resistance between the brush and the track will be considerable at other points, a very much lower contact resistance is obtained when the brush engages one of r I the ends 3 or the intermediate tap contact 50f silver-loaded plastic. The number of intermediate contacts provided may vary from case to case according to requirements.

,In the modified construction of Fig. 2, the conducting portions into which the terminals 4 extend, are formed as studs 3a, projecting from the lower side of the track 1a. This facilitates the manufacture of the track, and which may in this case wholly, including the studs 3a consist of carbon-loaded plastic.

In the embodiment of Fig. 3, corresponding parts have received reference numbers which are the same as in Fig. 1 preceded, however, by a 1, the track, for example, bearing the reference number 11. This track 11 is enclosed in the base 12 so as to be covered by insulating plastic 13a, only the end contacts 13 and .the intermediate contacts 15 being exposed at the surface.

Fig. 4 similarly illustrates a commutator switch, in which the fixed contacts '23 are formed of plastic rendered highly conductive by the incorporation of silver powder, their plastic matrix being made integral with the plastic constituting the insulating base 22. The terminals 24 project into their associated contacts 23, into which they are embedded by the moulding and curing operations. The wiper contact 26, which is mounted on a wiper arm 28, may advantageously consist of material similar to that of the fixed contacts 23. The fact that the matrix of the fixed contacts 23 is integral with the base body 22, ensures particularly smooth transition from each contact to the moulding surface, the risk of burning away the contact edges being greatly reduced compared with earlier constructions.

While the embodiments so far described were assumed, for the sake of simplicity of illustration, to have rectilinear tracks, Figs. to 8 illustrate embodiments with a part-circular track.

Figures 5 and 6 illustrate one form of device which may be conveniently used as a stepped attenuator, for example for use with a loud-speaker. The device comprises a moulded base 32 of a phenolformaldehyde plastic, on which a part circular resistor track 31 of a suitably resistive mixture of phenol-formaldehyde moulding powder and carbon powder is secured. The base is formed with an annular recess 40 surrounding a central hole 41 and with a number of contact-receiving recesses 42 which extend radially from a point near theouter periphery of the track to a point between the hole 41 and the inner edge of the resistor track 31. These recesses 42 serve to accommodate silver-loaded contact elements 33. A contact ring 43 of similar silver-loaded moulding material is formed in an annular recess 40. A shaft 44 is, by means of a bushing 46, rotatably carried in a cover plate 49, which is carried on the base 32 through a spacer-ring 50 and bolts and nuts 51 and 52, and this shaft carries, fixed at its end facing the base 32, a laterally projecting arm 47 having an elongated aperture 48. The latter guides a resilient contact member 38 formed with two contact ends 36a and 36 which respectively move on the contact ring 43 and on a smooth track formed by the flush upper surfaces of the base 32 and the contact elements 33. Y

. Interposed between the arm 47 and the end of the bushing 46 is a dished metal plate 53, which is firmly keyed to the shaft against rotation, and which in its turn imparts rotation to the arm 47 by means of a projection 54 of the latter which "fits into a suitable aperture of the plate 53. A retaining member 45 co-operates with the plate 53 in locating the shaft 44 against axial displacement'in the bushing 46. Means are preferably provided for .yieldingl'y indexing the spindle in the positions in which the contact-36 co-operates with one of the contac'ts 33, and for this purpose a spring spider 55 may be arranged to rotate with the dished member 53 and carry balls 56 which engage depressions 57 formed in the top member 49 at appropriate positions.

The joint final moulding operation of the base 32 and parts 31, 33, and 43 will ensure that the surfaces of the highly conductive contact element 33 and contact ring 43 are completely fiush with the upper surface of the insulating body 32, while the resistance track 31, in the form of an incomplete circle, projects upwardly from this surface. The gap between the two ends of resistance member 31 can be closed by a fillet 58 of insulating plastic material, as indicated at Figure 6.

Figures 7 to 8 illustrate a modified base member 32 at the back or whichprc-vision is made for external connection to the various contact elements 33 of the track 31. For this purpose metal studs 59 are inserted in the base 32 before the moulding of the contact elements 33, said studs extending from the back of the base into the cavities 4f. behind the track 31. During the formation of the contact element 33 and the moulding of the latter and of the ring 31, the inner ends of the studs 5'9 become firmly embedded in the associated one of the con tact elements 33 so that through the latter they are reliably connected to the appropriate portions of the resistor track 31. A further stud 66), similar to the studs 59, is preferably arranged to penetrate into the contact ring 43 in the groove 43.

While silver has been named as a specific example of metal proposed to be used for the mixture with moulding powder and has been found highly satisfactory, the in vention is not limited to the use of silver. Other metals or alloys may he used which have sufficiently low electrical resistance values and which can be moulded in mixture with the employed synthetic-resin moulding powder without reacting in a manner unduly affecting the desirable conductive qualities of the contact material. Thus an alloy of lead and 20% tin may be employed with Bakelite (l1.T.M.) or similar moulding powder, where the somewhat poorer conductive qualities of the resulting contact materials are adequate.

What we claim is:

l. A variable resistor device, comprising an insulating base made of insulating synthetic-resin moulding material and having a surface formed with a plurality of recesses spaced along a linear path, a resistor track secured to the base and consisting of a moulded resistive mixture of a conductive material and a synthetic resin moulding material, a plurality of contact elements constituted by j a synthetic resin matrix integral with that of the track and integrally joined with'the synthetic-resin material of by the incorporation of contiguous metal particles, and being at least partly accommodated in one each of said rccessesso as to present a contact surface flush with the surface of the base along said linear path, a wiper con tact co-operable with said flush surfaces along said path, and means for moving the wiper contact along the path to establish selective contact with the track through said contact elements respectively.

2. A variable resistor device as claimed in claim I including indexing means for yieldingly retaining the wiper contact in the various selected positions at which said wiper contact engages one of the contacts.

References Cited in the file of this patent UNITED STATES PATENTS 2,269,136 Tellkamp Jan. 6, 1942 2,629,166 Marsten et al. Feb. 24, 1953 2,632,830 Aust et al. Mar. 24, 1953 2,638,523 Rubin May 12, 1953 2,744,988 Tierman May 8, 1956 2,745,931 Heibel May 15, 1956 2,796,504 Pritikin et al. June 18, 1957 2,798,139 Dickinson et al. July 2, 1957 FOREIGN PATENTS 767,579 Great Britain Feb. 6, 1957 the base, said contact elements being highly conductive 

1. A VARIABLE RESISTOR DEVICE, COMPRISING AN INSULATING BASE MADE OF INSULATING SYNTHETIC-RESIN MOULDING MATERIAL AND HAVING A SURFACE FORMED WITH A PLURALITY OF RECESSES SPACED ALONG A LINEAR PATH, A RESISTOR TRACK SECURED TO THE BASE AND CONSISTING OF A MOULDED RESISTIVE MIXTURE OF A CONDUCTIVE MATERIAL AND A SYNTHETIC RESIN MOULDING MATERIAL, A PLURALITY OF CONTACT ELEMENTS CONSTITUTED BY A SYNTHETIC RESIN MATRIX INTEGRAL WITH THAT OF THE TRACK AND INTEGRALLY JOINED WITH THE SYNTHETIC-RESIN MATERIAL OF THE BASE, SAID CONTACT ELEMENTS BEING HIGHLY CONDUCTIVE BY THE INCORPORATION OF CONTIGUOUS METAL PARTICLES, AND BEING AT LEAST PARTLY ACCOMMODATED IN ONE EACH OF SAID, RECESSES SO AS TO PRESENT A CONTACT SURFACE FLUSH WITH THE SURFACE OF THE BASE ALONG SAID LINEAR PATH, A WIPER CONTACT CO-OPERABLE WITH SAID FLUSH SURFACES ALONG SAID PATH, AND MEANS FOR MOVING THE WIPER CONTACT ALONG THE PATH TO ESTABLISH SELECTIVE CONTACT WITH THE TRACK THROUGH SAID CONTACT ELEMENTS RESPECTIVELY. 